This invention relates to an improved sludge removal system for use in the removal of settled sludge collected on the bottom of a circular clarifier.
In a typical sewage or industrial waste treatment system, a clarifier tank normally defines one stage in a multi-stage treatment operation. The clarifier tank receives a "mixed liquor" from an initial stage aeration tank. Solids are separated from the liquid by settling in the clarifier, and the liquid is removed as effluent from the system and disposed of. Sludge comprising the accumulated solids is ordinarily removed for use in the next aeration stage of the sewage treatment.
To collect the sludge for removal to the aforementioned subsequent aeration stage, it has heretofore been conventional to provide a sludge collection trough at the center of the tank. The floor slopes downwardly towards the center from the periphery of the tank in a generally inverted conical configuration. It is intended that the sludge move inwardly towards the sludge collection trough where the sludge is drawn off by suitable means. To assist the movement of the sludge on the clarifier floor, a sludge scraper rake arrangement is usually provided.
In recent years the art of sludge removal has progressed to the use of sludge removal apparatus which utilizes the influence of the hydrostatic head imposed by the mass of liquid in the tank to force the sludge through uprising tubes. One such prior design comprises a tapered ported conduit rotatably mounted to sweep the floor of the tank. The ports are spaced along the bottom of the conduit and sludge settling on the floor of the tank is forced by hydrostatic pressure through the ports and along the conduit to a central discharge point. The main disadvantage of such apparatus is its inability to control the rate of solids withdrawal at the various discharge ports in proportion to different solids concentrations across the floor of the tank as a result of varying rates of solids settling.
Another heretofore used sludge removal apparatus includes a sludge rake structure mounted for slow rotation about the tank axis. The rake is provided with a plurality of uprising draw-off pipes, supported and rotated about the center column by the rake structure, with their inlet ends adjacent the tank floor whereby sludge is forced upwardly through the conduits and is delivered to a common collecting sump for eventual removal from the clarifier. Separate valves are provided on each conduit for control of the sludge withdrawal rate. The main disadvantage of such a system is that the sludge withdrawal control is independent of the rate at which collected sludge is eventually discharged from the collecting sump. As a result, increased sludge production cannot be accommodated except by resort to adjustment of each valve.
A most recent sludge removal design attempts to solve the above mentioned problems by providing apparatus which includes a sludge rake provided for rotation about the tank axis. The rake is provided with a plurality of nonstructural sludge conveying conduits of varying lengths which extend generally radially outward at different elevations to preselected areas of the tank bottom. A separate sludge receiving sump is provided for receipt of the sludge directly from a corresponding sludge conduit. A rotating ring is provided adjacent the sludge receiving sumps for receiving sludge from the sumps through corresponding submerged ports located between the lower portions of the sumps and the ring. The bottoms of the sumps and the rings are positioned below the normal liquid level of the tank. Adjustment means are provided to each port for maintaining a normal sludge level in the corresponding sludge receiving sump above the level of the port. An example of such a design is disclosed in U.S. Pat. No. 3,298,529.
Although the above design has solved the hereinabove mentioned problem of accommodating a wide range of sludge withdrawal rates without frequent adjustment, it still lacks various features desired from the commercial standpoint. This is due to the fact that the sludge conveying conduits and sludge rake present a large surface area in vertical section which results in increased drag and consequently requires a large torque output from the drive mechanism. Along the same lines, this large cross sectional surface area tends to rotate the entire mass of the sludge blanket which significantly impedes the sludge removal process. Another disadvantage of the above design is that floating material and suspended solids tend to collect on the top surfaces of the sludge receiving sumps as a result of the ports always being submerged. This requires periodic cleaning to prevent the formation of an excessive build-up of what becomes an unsightly and smelly mess.